#!/usr/bin/env perl

sub char_sort {
        my %chars = (
                "a" => 1,
                "b" => 2,
                "c" => 3,
                "d" => 4,
                "e" => 5,
                "f" => 6,
                "g" => 7,
                "h" => 8,
                "i" => 9,
                "j" => 10,
                "k" => 11,
                "l" => 12,
                "m" => 13,
                "n" => 14,
                "o" => 15,
                "p" => 16,
                "q" => 17,
                "r" => 18,
                "s" => 19,
                "t" => 20,
                "u" => 21,
                "v" => 22,
                "w" => 23,
                "x" => 24,
                "y" => 25,
                "z" => 26);

        # perl sets $a and $b for the values to compare. 
        # This function itself uses itself and calls with two parameters
        # select which type of call and wich arguments to use
        $word_a = (length($_[0])!=0)?$_[0]:$a;
        $word_b = (length($_[1])!=0)?$_[1]:$b;

        # Get the first chars, which we need to compare
        $a1=substr($word_a,0,1);
        $b1=substr($word_b,0,1);

        # print("A1=$a1 B1=$b1 A=$word_a B=$word_b\n");

        # if both args are empty return with equality(0)
        return 0 if(length($word_a)==0 and length($word_b)==0);

        # if current char is equal, call this function with the substrings beginning at the second char
        return char_sort(substr($word_a,1),substr($word_b,1)) if (($chars{$a1} <=> $chars{$b1})==0);

        # if current char is different, we're finished now
        return $chars{$a1} <=> $chars{$b1};
}

@list = ("my","favorite","animal","book","for","advanced","biologists");
@sorted = sort char_sort @list;

print("\n");
print("*** Unsorted ***\n");
foreach(@list) {
        print;
        print("\n");
 }
print("\n");

print("*** Sorted ***\n");
foreach(@sorted) {
        print;
        print("\n");
 }
print("\n");